RFC 7422

Independent Submission C. Donley
Request for Comments: 7422 CableLabs
Category: Informational C. Grundemann
ISSN: 2070-1721 Internet Society
V. SarawatK. Sundaresan
CableLabs
O. Vautrin
Juniper Networks
December 2014 Deterministic Address Mapping to Reduce Logging in
Carrier-Grade NAT Deployments
Abstract
In some instances, Service Providers (SPs) have a legal logging
requirement to be able to map a subscriber's inside address with the
address used on the public Internet (e.g., for abuse response).
Unfortunately, many logging solutions for Carrier-Grade NATs (CGNs)
require active logging of dynamic translations. CGN port assignments
are often per connection, but they could optionally use port ranges.
Research indicates that per-connection logging is not scalable in
many residential broadband services. This document suggests a way to
manage CGN translations in such a way as to significantly reduce the
amount of logging required while providing traceability for abuse
response. IPv6 is, of course, the preferred solution. While
deployment is in progress, SPs are forced by business imperatives to
maintain support for IPv4. This note addresses the IPv4 part of the
network when a CGN solution is in use.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7422.

Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction ....................................................21.1. Requirements Language ......................................42. Deterministic Port Ranges .......................................42.1. IPv4 Port Utilization Efficiency ...........................72.2. Planning and Dimensioning ..................................72.3. Deterministic CGN Example ..................................83. Additional Logging Considerations ...............................93.1. Failover Considerations ...................................104. Impact on the IPv6 Transition ..................................105. Privacy Considerations .........................................116. Security Considerations ........................................117. References .....................................................117.1. Normative References ......................................117.2. Informative References ....................................12
Acknowledgements ..................................................13
Authors' Addresses ................................................141. Introduction
It is becoming increasingly difficult to obtain new IPv4 address
assignments from Regional/Local Internet Registries due to depleting
supplies of unallocated IPv4 address space. To meet the growing
demand for Internet connectivity from new subscribers, devices, and
service types, some operators will be forced to share a single public
IPv4 address among multiple subscribers using techniques such as
Carrier-Grade NAT (CGN) [RFC6264] (e.g., NAT444 [NAT444], Dual-Stack
Lite (DS-Lite) [RFC6333], NAT64 [RFC6146], etc.). However, address
sharing poses additional challenges to operators when considering how
they manage service entitlement, public safety requests, or
attack/abuse/fraud reports [RFC6269]. In order to identify a
specific user associated with an IP address in response to such a
request or for service entitlement, an operator will need to map a
subscriber's internal source IP address and source port with the

global public IP address and source port provided by the CGN for
every connection initiated by the user.
CGN connection logging satisfies the need to identify attackers and
respond to abuse/public safety requests, but it imposes significant
operational challenges to operators. In lab testing, we have
observed CGN log messages to be approximately 150 bytes long for
NAT444 [NAT444] and 175 bytes for DS-Lite [RFC6333] (individual log
messages vary somewhat in size). Although we are not aware of
definitive studies of connection rates per subscriber, reports from
several operators in the US sets the average number of connections
per household at approximately 33,000 connections per day. If each
connection is individually logged, this translates to a data volume
of approximately 5 MB per subscriber per day, or about 150 MB per
subscriber per month; however, specific data volumes may vary across
different operators based on myriad factors. Based on available
data, a 1-million-subscriber SP will generate approximately 150
terabytes of log data per month, or 1.8 petabytes per year. Note
that many SPs compress log data after collection; compression factors
of 2:1 or 3:1 are common.
The volume of log data poses a problem for both operators and the
public safety community. On the operator side, it requires a
significant infrastructure investment by operators implementing CGN.
It also requires updated operational practices to maintain the
logging infrastructure, and requires approximately 23 Mbps of
bandwidth between the CGN devices and the logging infrastructure per
50,000 users. On the public safety side, it increases the time
required for an operator to search the logs in response to an abuse
report, and it could delay investigations. Accordingly, an
international group of operators and public safety officials
approached the authors to identify a way to reduce this impact while
improving abuse response.
The volume of CGN logging can be reduced by assigning port ranges
instead of individual ports. Using this method, only the assignment
of a new port range is logged. This may massively reduce logging
volume. The log reduction may vary depending on the length of the
assigned port range, whether the port range is static or dynamic,
etc. This has been acknowledged in [RFC6269], which recommends the
logging of source ports at the server and/or destination logging at
the CGN, and [NAT-LOGGING], which describes information to be logged
at a NAT.
However, the existing solutions still pose an impact on operators and
public safety officials for logging and searching. Instead, CGNs
could be designed and/or configured to deterministically map internal
addresses to {external address + port range} in such a way as to be

able to algorithmically calculate the mapping. Only inputs and
configuration of the algorithm need to be logged. This approach
reduces both logging volume and subscriber identification times. In
some cases, when full deterministic allocation is used, this approach
can eliminate the need for translation logging.
This document describes a method for such CGN address mapping,
combined with block port reservations, that significantly reduces the
burden on operators while offering the ability to map a subscriber's
inside IP address with an outside address and external port number
observed on the Internet.
The activation of the proposed port range allocation scheme is
compliant with BEHAVE requirements such as the support of
Application-specific functions (APP).
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Deterministic Port Ranges
While a subscriber uses thousands of connections per day, most
subscribers use far fewer resources at any given time. When the
compression ratio (see Appendix B of RFC 6269 [RFC6269]) is low
(e.g., the ratio of the number of subscribers to the number of public
IPv4 addresses allocated to a CGN is closer to 10:1 than 1000:1),
each subscriber could expect to have access to thousands of TCP/UDP
ports at any given time. Thus, as an alternative to logging each
connection, CGNs could deterministically map customer private
addresses (received on the customer-facing interface of the CGN,
a.k.a., internal side) to public addresses extended with port ranges
(used on the Internet-facing interface of the CGN, a.k.a., external
side). This algorithm allows an operator to identify a subscriber
internal IP address when provided the public side IP and port number
without having to examine the CGN translation logs. This prevents an
operator from having to transport and store massive amounts of
session data from the CGN and then process it to identify a
subscriber.
The algorithmic mapping can be expressed as:
(External IP Address, Port Range) = function 1 (Internal IP Address)
Internal IP Address = function 2 (External IP Address, Port Number)

The CGN SHOULD provide a method for administrators to test both
mapping functions (e.g., enter an External IP Address + Port Number
and receive the corresponding Internal IP Address).
Deterministic Port Range allocation requires configuration of the
following variables:
o Inside IPv4/IPv6 address range (I);
o Outside IPv4 address range (O);
o Compression ratio (e.g., inside IP addresses I / outside IP
addresses O) (C);
o Dynamic address pool factor (D), to be added to the compression
ratio in order to create an overflow address pool;
o Maximum ports per user (M);
o Address assignment algorithm (A) (see below); and
o Reserved TCP/UDP port list (R)
Note: The inside address range (I) will be an IPv4 range in NAT444
operation (NAT444 [NAT444]) and an IPv6 range in DS-Lite operation
(DS-Lite [RFC6333]).
A subscriber is identified by an internal IPv4 address (e.g., NAT44)
or an IPv6 prefix (e.g., DS-Lite or NAT64).
The algorithm may be generalized to L2-aware NAT [L2NAT], but this
requires the configuration of the Internal interface identifiers
(e.g., Media Access Control (MAC) addresses).
The algorithm is not designed to retrieve an internal host among
those sharing the same internal IP address (e.g., in a DS-Lite
context, only an IPv6 address/prefix can be retrieved using the
algorithm while the internal IPv4 address used for the encapsulated
IPv4 datagram is lost).
Several address-assignment algorithms are possible. Using predefined
algorithms, such as those that follow, simplifies the process of
reversing the algorithm when needed. However, the CGN MAY support
additional algorithms. Also, the CGN is not required to support all
of the algorithms described below. Subscribers could be restricted

to ports from a single IPv4 address or could be allocated ports
across all addresses in a pool, for example. The following
algorithms and corresponding values of A are as follows:
0: Sequential (e.g., the first block goes to address 1, the second
block to address 2, etc.).
1: Staggered (e.g., for every n between 0 and ((65536-R)/(C+D))-1 ,
address 1 receives ports n*C+R, address 2 receives ports
(1+n)*C+R, etc.).
2: Round robin (e.g., the subscriber receives the same port number
across a pool of external IP addresses. If the subscriber is to
be assigned more ports than there are in the external IP pool, the
subscriber receives the next highest port across the IP pool, and
so on. Thus, if there are 10 IP addresses in a pool and a
subscriber is assigned 1000 ports, the subscriber would receive a
range such as ports 2000-2099 across all 10 external IP
addresses).
3: Interlaced horizontally (e.g., each address receives every Cth
port spread across a pool of external IP addresses).
4: Cryptographically random port assignment (Section 2.2 of RFC6431
[RFC6431]). If this algorithm is used, the SP needs to retain the
keying material and specific cryptographic function to support
reversibility.
5: Vendor-specific. Other vendor-specific algorithms may also be
supported.
The assigned range of ports MAY also be used when translating ICMP
requests (when rewriting the Identifier field).
The CGN then reserves ports as follows:
1. The CGN removes reserved ports (R) from the port candidate list
(e.g., 0-1023 for TCP and UDP). At a minimum, the CGN SHOULD
remove system ports [RFC6335] from the port candidate list
reserved for deterministic assignment.
2. The CGN calculates the total compression ratio (C+D), and
allocates 1/(C+D) of the available ports to each internal IP
address. Specific port allocation is determined by the algorithm
(A) configured on the CGN. Any remaining ports are allocated to
the dynamic pool.

Note: Setting D to 0 disables the dynamic pool. This option
eliminates the need for per-subscriber logging at the expense of
limiting the number of concurrent connections that 'power users'
can initiate.
3. When a subscriber initiates a connection, the CGN creates a
translation mapping between the subscriber's inside local IP
address/port and the CGN outside global IP address/port. The CGN
MUST use one of the ports allocated in step 2 for the translation
as long as such ports are available. The CGN SHOULD allocate
ports randomly within the port range assigned by the
deterministic algorithm. This is to increase subscriber privacy.
The CGN MUST use the pre-allocated port range from step 2 for
Port Control Protocol (PCP, [RFC6887]) reservations as long as
such ports are available. While the CGN maintains its mapping
table, it need not generate a log entry for translation mappings
created in this step.
4. If D>0, the CGN will have a pool of ports left for dynamic
assignment. If a subscriber uses more than the range of ports
allocated in step 2 (but fewer than the configured maximum ports
M), the CGN assigns a block of ports from the dynamic assignment
range for such a connection or for PCP reservations. The CGN
MUST log dynamically assigned port blocks to facilitate
subscriber-to-address mapping. The CGN SHOULD manage dynamic
ports as described in [LOG-REDUCTION].
5. Configuration of reserved ports (e.g., system ports) is left to
operator configuration.
Thus, the CGN will maintain translation mapping information for all
connections within its internal translation tables; however, it only
needs to externally log translations for dynamically assigned ports.
2.1. IPv4 Port Utilization Efficiency
For SPs requiring an aggressive address-sharing ratio, the use of the
algorithmic mapping may impact the efficiency of the address sharing.
A dynamic port range allocation assignment is more suitable in those
cases.
2.2. Planning and Dimensioning
Unlike dynamic approaches, the use of the algorithmic mapping
requires more effort from operational teams to tweak the algorithm
(e.g., size of the port range, address sharing ratio, etc.).
Dedicated alarms SHOULD be configured when some port utilization
thresholds are fired so that the configuration can be refined.

Subscriber 2 concurrently uses more than the allocated 4032 ports
(e.g., for peer-to-peer, mapping, video streaming, or other
connection-intensive traffic types), the CGN allocates up to an
additional 1008 ports using bulk port reservations. In this example,
subscriber 2 uses outside ports 5056-9087, and then 100-port blocks
between 58000-58999. Connections using ports 5056-9087 are not
logged, while 10 log entries are created for ports 58000-58099,
58100-58199, 58200-58299, ..., 58900-58999.
In order to identify a subscriber behind a CGN (regardless of port
allocation method), public safety agencies need to collect source
address and port information from content provider log files. Thus,
content providers are advised to log source address, source port, and
timestamp for all log entries, per [RFC6302]. If a public safety
agency collects such information from a content provider and reports
abuse from 192.0.2.1, port 2001, the operator can reverse the mapping
algorithm to determine that the internal IP address subscriber 1 has
been assigned generated the traffic without consulting CGN logs (by
correlating the internal IP address with DHCP/PPP lease connection
records). If a second abuse report comes in for 192.0.2.1, port
58204, the operator will determine that port 58204 is within the
dynamic pool range, consult the log file, correlate with connection
records, and determine that subscriber 2 generated the traffic
(assuming that the public safety timestamp matches the operator
timestamp. As noted in RFC 6292 [RFC6292], accurate timekeeping
(e.g., use of NTP or Simple NTP) is vital).
In this example, there are no log entries for the majority of
subscribers, who only use pre-allocated ports. Only minimal logging
would be needed for those few subscribers who exceed their pre-
allocated ports and obtain extra bulk port assignments from the
dynamic pool. Logging data for those users will include inside
address, outside address, outside port range, and timestamp.
Note that in a production environment, operators are encouraged to
consider [RFC6598] for assigning inside addresses.
3. Additional Logging Considerations
In order to be able to identify a subscriber based on observed
external IPv4 address, port, and timestamp, an operator needs to know
how the CGN was configured with regard to internal and external IP
addresses, dynamic address pool factor, maximum ports per user, and
reserved port range at any given time. Therefore, the CGN MUST
generate a record any time such variables are changed. The CGN
SHOULD generate a log message any time such variables are changed.
The CGN MAY keep such a record in the form of a router configuration
file. If the CGN does not generate a log message, it would be up to

the operator to maintain version control of router config changes.
Also, the CGN SHOULD generate such a log message once per day to
facilitate quick identification of the relevant configuration in the
event of an abuse notification.
Such a log message MUST, at minimum, include the timestamp, inside
prefix I, inside mask, outside prefix O, outside mask, D, M, A, and
reserved port list R; for example:
[Wed Oct 11 14:32:52
2000]:198.51.100.0:28:192.0.2.0:32:2:5040:0:1-1023,5004,5060.
3.1. Failover Considerations
Due to the deterministic nature of algorithmically assigned
translations, no additional logging is required during failover
conditions provided that inside address ranges are unique within a
given failover domain. Even when directed to a different CGN server,
translations within the deterministic port range on either the
primary or secondary server can be algorithmically reversed, provided
the algorithm is known. Thus, if 198.51.100.1 port 3456 maps to
192.0.2.1 port 1000 on CGN 1 and 198.51.100.1 port 1000 on Failover
CGN 2, an operator can identify the subscriber based on outside
source address and port information.
Similarly, assignments made from the dynamic overflow pool need to be
logged as described above, whether translations are performed on the
primary or failover CGN.
4. Impact on the IPv6 Transition
The solution described in this document is applicable to CGN
transition technologies (e.g., NAT444, DS-Lite, and NAT64). As
discussed in [RFC7021], the authors acknowledge that native IPv6 will
offer subscribers a better experience than CGN. However, many
Customer Premises Equipment (CPE) devices only support IPv4.
Likewise, as of October 2014, only approximately 5.2% of the top 1
million websites were available using IPv6. Accordingly,
Deterministic CGN should in no way be understood as making CGN a
replacement for IPv6 service; however, until such time as IPv6
content and devices are widely available, Deterministic CGN will
provide operators with the ability to quickly respond to public
safety requests without requiring excessive infrastructure,
operations, and bandwidth to support per-connection logging.

5. Privacy Considerations
The algorithm described above makes it easier for SPs and public
safety officials to identify the IP address of a subscriber through a
CGN system. This is the equivalent level of privacy users could
expect when they are assigned a public IP address and their traffic
is not translated. However, this algorithm could be used by other
actors on the Internet to map multiple transactions to a single
subscriber, particularly if ports are distributed sequentially.
While still preserving traceability, subscriber privacy can be
increased by using one of the other values of the Address Assignment
Algorithm (A), which would require interested parties to know more
about the Service Provider's CGN configuration to be able to tie
multiple connections to a particular subscriber.
6. Security Considerations
The security considerations applicable to NAT operation for various
protocols as documented in, for example, RFC 4787 [RFC4787] and RFC
5382 [RFC5382] also apply to this document.
Note that, with the possible exception of cryptographically based
port allocations, attackers could reverse-engineer algorithmically
derived port allocations to either target a specific subscriber or to
spoof traffic to make it appear to have been generated by a specific
subscriber. However, this is exactly the same level of security that
the subscriber would experience in the absence of CGN. CGN is not
intended to provide additional security by obscurity.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007,
<http://www.rfc-editor.org/info/rfc4787>.
[RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, October 2008,
<http://www.rfc-editor.org/info/rfc5382>.